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Biogeochemistry of trace metals in European shelf seas

Biogeochemistry of trace metals in European shelf seas
Biogeochemistry of trace metals in European shelf seas
Shelf seas are considered an important source of trace metals (TMs) to the open ocean. However, the processes controlling distributions of dissolved TMs (DTMs) in shelf seas, their seasonal variability and the mechanisms facilitating off-­‐shelf transport to the open ocean are not well constrained. Reported here is the first seasonal study of the DTMs, cadmium (DCd), zinc (DZn), nickel (DNi) copper (DCu) manganese (DMn), cobalt (DCo), aluminium (DAl) and lead (DPb) and their contrasting biogeochemical behaviours in the Celtic Sea continental system. Strong correlations (r2 = 0.91 – 0.97, n = 454) between nutrient-­‐like metals (DCd, DZn, DCu, DNi, DCo (surface waters only)) and macronutrients (phosphate (PO43-­‐) and silicic acid (SiO44-­‐) were observed over all seasons (autumn, spring and summer) indicating that biological uptake in the euphotic zone and remineralization of sinking phytoplankton debris in deeper waters form key controls on their distributions. The correlation between DAl and PO43-­‐ (r2 = 0.80) and SiO44-­‐ (r2 = 0.85) in the upper waters (< 1500 m depth) mimicked the DZn distributions, which was a surprising observation and requires further investigation. Variable DTM ratios relative to PO43-­‐ were observed across different seasons in off-­‐shelf surface waters and for DCd and DZn (full depth profiles) across the continental shelf. Observations from the Celtic Sea region suggest the influence of distinct water masses, phytoplankton community structure and potentially metal specific biogeochemical processes, on DTMs distributions on a seasonal time-­‐scale. Dissolved Co and Mn were more strongly influenced by scavenging and sedimentary inputs. Vertical distributions of DMn showed a typical scavenged type behaviour with enhanced surface waters concentrations from atmospheric inputs combined with photochemical stabilisation. Enhanced DMn and DCo concentrations were observed close to slope sediments and extended away from the shelf at intermediate depths. These observations were related to suspended sediments as indicated by increased signals in turbidity and short-­‐lived 223Ra and 224Ra isotopes (tracers of sediment inputs). This implies that resuspended shelf or slope sediments can act as a source of DMn and DCo that can be transported to the adjacent open ocean. The full depth relationship of DCo with PO43-­‐ revealed a shift in dominant biogeochemical processes controlling DCo vertical distributions in the study region. While biological uptake and remineralization were dominant processes in surface waters, as indicated by a strong correlation with PO43-­‐ (r = 0.89), a gradual decoupling of DCo from PO43-­‐ at intermediate depths was suggestive of progressive scavenging and less pronounced remineralization and a significant negative correlation (r = -­‐0.72) in deeper waters (> 1500 m depth) was suggestive of scavenging as the dominant process at these depths. Beside the natural sources of DTMs, anthropogenic activities have resulted in enhanced DTMs emissions to the environment over the past centuries. Lead has a strong anthropogenic signal as a result of the combustion of leaded fuel and coal. Presented here is the first combined DPb, labile Pb (LpPb) and particulate Pb (PPb) distributions from the Celtic Sea since the phasing out of leaded fuel in Europe. Concentrations of DPb in surface waters have decreased by 4-­‐fold over the last four decades. Nevertheless, a distinct anthropogenic Pb signal was observed from the Mediterranean Sea at intermediate depths, showing that Pb can transported over long distances (>2500 km). Benthic DPb fluxes exceeded the atmospheric Pb fluxes in the region, indicating the importance of sediments as a contemporary Pb source. A strong positive correlation between DPb, PPb and LpPb indicates a dynamic equilibrium between the phases and the potential for particles to ‘buffer’ the DPb pool. This study highlights the requirements of continuing efforts in regulating Pb emissions but also of detailed seasonal TMs studies in coastal systems in future. Results reported here provide insights into distinct biogeochemical processes in oceanographically dynamic shelf seas and demonstrates the potential of Pb in constraining ocean circulation patterns.
University of Southampton
Rusiecka, Dagmara
70c9f91b-6ed5-451a-bccb-832fb7022724
Rusiecka, Dagmara
70c9f91b-6ed5-451a-bccb-832fb7022724
Achterberg, Eric
685ce961-8c45-4503-9f03-50f6561202b9

Rusiecka, Dagmara (2018) Biogeochemistry of trace metals in European shelf seas. University of Southampton, Doctoral Thesis, 188pp.

Record type: Thesis (Doctoral)

Abstract

Shelf seas are considered an important source of trace metals (TMs) to the open ocean. However, the processes controlling distributions of dissolved TMs (DTMs) in shelf seas, their seasonal variability and the mechanisms facilitating off-­‐shelf transport to the open ocean are not well constrained. Reported here is the first seasonal study of the DTMs, cadmium (DCd), zinc (DZn), nickel (DNi) copper (DCu) manganese (DMn), cobalt (DCo), aluminium (DAl) and lead (DPb) and their contrasting biogeochemical behaviours in the Celtic Sea continental system. Strong correlations (r2 = 0.91 – 0.97, n = 454) between nutrient-­‐like metals (DCd, DZn, DCu, DNi, DCo (surface waters only)) and macronutrients (phosphate (PO43-­‐) and silicic acid (SiO44-­‐) were observed over all seasons (autumn, spring and summer) indicating that biological uptake in the euphotic zone and remineralization of sinking phytoplankton debris in deeper waters form key controls on their distributions. The correlation between DAl and PO43-­‐ (r2 = 0.80) and SiO44-­‐ (r2 = 0.85) in the upper waters (< 1500 m depth) mimicked the DZn distributions, which was a surprising observation and requires further investigation. Variable DTM ratios relative to PO43-­‐ were observed across different seasons in off-­‐shelf surface waters and for DCd and DZn (full depth profiles) across the continental shelf. Observations from the Celtic Sea region suggest the influence of distinct water masses, phytoplankton community structure and potentially metal specific biogeochemical processes, on DTMs distributions on a seasonal time-­‐scale. Dissolved Co and Mn were more strongly influenced by scavenging and sedimentary inputs. Vertical distributions of DMn showed a typical scavenged type behaviour with enhanced surface waters concentrations from atmospheric inputs combined with photochemical stabilisation. Enhanced DMn and DCo concentrations were observed close to slope sediments and extended away from the shelf at intermediate depths. These observations were related to suspended sediments as indicated by increased signals in turbidity and short-­‐lived 223Ra and 224Ra isotopes (tracers of sediment inputs). This implies that resuspended shelf or slope sediments can act as a source of DMn and DCo that can be transported to the adjacent open ocean. The full depth relationship of DCo with PO43-­‐ revealed a shift in dominant biogeochemical processes controlling DCo vertical distributions in the study region. While biological uptake and remineralization were dominant processes in surface waters, as indicated by a strong correlation with PO43-­‐ (r = 0.89), a gradual decoupling of DCo from PO43-­‐ at intermediate depths was suggestive of progressive scavenging and less pronounced remineralization and a significant negative correlation (r = -­‐0.72) in deeper waters (> 1500 m depth) was suggestive of scavenging as the dominant process at these depths. Beside the natural sources of DTMs, anthropogenic activities have resulted in enhanced DTMs emissions to the environment over the past centuries. Lead has a strong anthropogenic signal as a result of the combustion of leaded fuel and coal. Presented here is the first combined DPb, labile Pb (LpPb) and particulate Pb (PPb) distributions from the Celtic Sea since the phasing out of leaded fuel in Europe. Concentrations of DPb in surface waters have decreased by 4-­‐fold over the last four decades. Nevertheless, a distinct anthropogenic Pb signal was observed from the Mediterranean Sea at intermediate depths, showing that Pb can transported over long distances (>2500 km). Benthic DPb fluxes exceeded the atmospheric Pb fluxes in the region, indicating the importance of sediments as a contemporary Pb source. A strong positive correlation between DPb, PPb and LpPb indicates a dynamic equilibrium between the phases and the potential for particles to ‘buffer’ the DPb pool. This study highlights the requirements of continuing efforts in regulating Pb emissions but also of detailed seasonal TMs studies in coastal systems in future. Results reported here provide insights into distinct biogeochemical processes in oceanographically dynamic shelf seas and demonstrates the potential of Pb in constraining ocean circulation patterns.

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Rusiecka, Dagmara_PhD_Thesis_2018 - Author's Original
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Published date: 25 September 2018

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Local EPrints ID: 425510
URI: http://eprints.soton.ac.uk/id/eprint/425510
PURE UUID: b4b45ff0-4e34-46c5-b362-a39384ae8434

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Date deposited: 22 Oct 2018 16:30
Last modified: 13 Mar 2019 17:56

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